[SI-LIST] Re: Diff.Pairs

From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>

Date: Mon, 6 Oct 2003 15:16:55 -0700

The main reason why you have to neck down your differential trace is to
maintain your single end signal impedance. If you only have to worry about
differential impedance, you can define a stack up that can have reasonable
differential coupling and trace width with thicker dielectric. There are
many designs such as FCAL or 3GIO clusters that have only differential
critical nets. It is relatively easy to define a stackup for optimizing
differential traces in those application without worrying about traces being
too narrow and skin effects. The problem arise when you mix single ended
CMOS technology (in typical CPU,memory or I/O buses) with the differential
signals. Then it will be hard to not narrowing differential traces or making
single ended line too wide and lose the routing efficiency.
-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
Sent: Saturday, October 04, 2003 11:38 AM
To: Chris Cheng; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Diff.Pairs
I've had both on the same design for many years. Don't get you point.
> [Original Message]
> From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 10/3/2003 1:03:30 PM
> Subject: [SI-LIST] Re: Diff.Pairs
>
> I thought you are old enough to remember the days when board traces are
wide
> (6-8 mils were state of the art) and technology don't mix (either you have
> ECL or CMOS as critical highspeed design on the system but not both). It
is
> easy to justify tight coupling of different pairs then. It still makes
sense
> for many designs where differential I/O is the only thing you care (e.g.
> FCAL or 3GIO subsystems or clusters).
>
> -----Original Message-----
> From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> Sent: Friday, October 03, 2003 11:41 AM
> To: Duane Takahashi; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Diff.Pairs
>
>
> More than that, it does not have any benefit. Tight coupling of
> differential pairs forces the traces to be narrower increasing the skin
> effect losses. Also, this tight coupling is going to result in good old
> cross talk that actually degrades the edges.
>
> How the notion of tight coupling of differential pairs as beneficial got
> started is a mystery to me. There are several references that show that
> tight coupling is not beneficial, one of them is Howard Johnson's latest
> book, at least one column he has written and my recently released book.
>
> Lee Ritchey
>
>
>
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